1. Field of the Invention
The present invention relates to zinc-oxide surge arresters, and more particularly, to a ZnO surge arrester applicable to operation where the maximum operating temperature is higher than 125° C.
2. Description of Prior Art
ZnO surge arrester is an impedance element whose resistance varies non-linearly with voltages, and is mainly made of zinc oxide powder sintered with metallic oxide additives, such as Bi2O3, Sb2O3, CaO, Cr2O3, Co2O3 and MnO, into sintered ceramic at high temperature. For enhancing the sintering properties of the material, a small amount of SiO2 may be also added.
Such a ZnO surge arrester possesses excellent non-ohmic characteristics and good capability of surge absorption, while having a desirable nonlinear I-V characteristic curve. Since its resistance is high when the voltage is low, and when the voltage is high, its resistance decreases sharply, it is also referred to as a varistor.
ZnO surge arresters are often used to protect electronic circuits from damage or interference caused by excessively high transient voltages. In normal operational conditions, a surge arrester staying standby presents high impedance (megohms) with respect to the electronic components it protects, and thus forces currents to proceed along the designed path instead of passing therethrough, thereby maintaining the circuit properties as designed. In case of a transient voltage surge that is higher than the breakdown voltage of the surge arrester, the surge arrester has its impedance lowered to a few ohms, so as to allow the surge voltage to pass therethrough in a short-circuit-like state, and thereby shunt the current to ground elements, thereby protecting electronic products or expensive circuit components from being damaged by the surge.
Those surge arresters applied to common information products for the purposes of voltage stabilization and surge absorption typically endure a maximum operating temperature up to about 85° C. However, with the fast development of electronic products and communication products, the requirements for heat resistance of surge arresters are becoming stricter. For example, surge arresters applied to electronic circuits of ABS (Antilock Brake System), airbags or power steering wheels for automobiles have to work in an operating temperature higher than 125° C., or even higher than 150° C. Nevertheless, in the state-of-the-art technology, there has not been any ZnO surge arrester capable of working at 150° C. proposed.
In addition, in the sintered ceramic of the existing ZnO surge arresters, the grain boundary layer between ZnO grains is typically made of NTC (Negative Temperature Coefficient) thermistor materials whose resistance reduces with raising temperature, and when the working temperature of the existing ZnO surge arresters raises, the current carriers in the materials of the grain boundary layer of the existing ZnO surge arresters move in a higher mobility. With the impact of the working voltage, the existing ZnO surge arrester shows a decrease in breakdown voltage, resistance and nonlinear exponent, and an increase in leakage current, thus deteriorating. Consequently, the ZnO surge arrester can be burned out.
Hence, it is desired to have a ZnO surge arrester applicable in an operating temperature higher than 125° C. The present invention thus proposes a solution that is to add a PTC (Positive Temperature Coefficient) thermistor material in the grain boundary layer between ZnO grains in a ZnO surge arrester, so that when the working temperature raises, the PTC thermistor material has its resistance sharply increased for compensating or partially compensating the resistance of the traditional materials in the grain boundary layer reduced due to the increased temperature. Thereby, the grain boundary layer in the ZnO surge arrester can have its resistance more independent of temperature, so as to significantly improve the ZnO surge arrester in capability of enduring high-temperature operation.